But in May, Chrome gained 1.3 percentage points, more than double its average increase over the last 12 months, to climb to 20.2%, while Firefox lost six-tenths of a point to fall to 19.6%.

Last month was the first time that Chrome cracked the 20% mark -- the browser debuted in September 2008 -- and the first time that Firefox fell under that number in Net Applications' data since October of the same year.

Chrome Steals Second From Firefox in Browser WarsFirefox, backed by open-source developer Mozilla, peaked at just over 25% in April 2010, and has been on a slow-but-steady decline in usage share since then.

For Microsoft, May was a return to a more traditional pattern: Internet Explorer (IE) lost half a percentage point to end the month at 53.6%. May's decline put an end to the two-month-in-a-row growth IE had experienced, and returned the browser to near the share it owned last March.

Even so, IE has gained share in three of the first five months of 2012.

Within the IE family, IE9 continued its ascent, adding one percentage point to account for 16.9% of all browsers on all operating systems. IE8 also was up, boosting its share by nearly half a point to 26.7%.

The other editions -- 2006's IE7 and the 11-year-old IE6 -- lost share in May. IE6, the version Microsoft wants to disappear, lost a point last month, falling to 6.1%, a record low in Net Applications' tracking. IE7 shed seven-tenths of a percentage point to drop to 3.4%, also a record.

While the shift toward IE9 can be attributed to the increasing uptake of Windows 7, IE8's recent rebound is harder to explain. IE8 has grown its share in four of the first five months of the year compared to only two such months during all of 2011.

The shift toward IE8 and the above-average declines of both IE6 and IE7 so far this year may be due to Microsoft's new practice of automatically upgrading older versions. Late last year, the company said it would begin to silently force Windows to upgrade IE to the newest-possible edition, ending a tradition of asking users' permission for such moves.

In January, Microsoft started upgrading some PCs running Windows XP from IE6 or IE7 to IE8, and swapping IE9 for IE7 or IE8 on Vista and Windows 7.

The process started in Australia and Brazil, and is to gradually roll out worldwide this year. Microsoft has declined to provide the names of countries where it has switched on the silent IE upgrades.

Apple's Safari lost two-tenths of a point last month to end at 4.6%, while Opera Software's Opera was flat at 1.6%.

StatCounter's calculations, however, were considerably different than Net Applications', as they tend to be.

Net Applications had IE falling by almost two percentage points to 32.1%, while Chrome grew by 1.2 percentage points to 32.4%, making good on reports throughout May that showed Chrome would kick IE out of first place. Firefox, said StatCounter, climbed to 25.6%, while Safari and Opera didn't budge, accounting for shares of 7.1% and 1.7%, respectively.

Net Applications calculates browser usage share with data obtained from more than 160 million unique visitors who browse 40,000 Web sites that the company monitors. More browser share figures can be found on the company's site.